I have started rewriting my generation ship story and the first scene after the background info(just a little to start it up, not a lot) is where an alien named Pistol Star decides who is to go on the generation ship. He uses very specific criteria to narrow things down. Here is the not allowed list:

  • Overweight and Obese
  • Underweight
  • Pregnant
  • Below 18
  • Orphaned
  • Above 50
  • Severe illness
  • Disease other than illness(diabetic, heart disease etc.)

After all the narrowing, 5 thousand people in an equal male to female ratio are chosen to maximize diversity while having an easy to control population.

The aliens do allow some ill people on board such as those sick with the cold or the flu or a stomach virus. Even some pneumonia is allowed on board but only the milder viral pneumonia.

Is the narrowing too restrictive?

Note: The aliens view pregnancy before survival training for a given generation of humans is finished as a bad thing and will kill pregnant women that haven't finished survival training and don't allow pregnant women on board at the beginning when they are on earth and the narrowing down is going on.


What I mean by too restrictive is will it reduce the population by too much in terms of diversity? And there are plenty of people on earth that would love to adopt an orphan and care for it and will do it and orphans will probably regret more being in space than being an orphan.

  • $\begingroup$ Why no one below 18? I can understand not having children but surely 15-30 year olds are the healthiest and best able to survive. $\endgroup$ Nov 20, 2016 at 18:07
  • $\begingroup$ Well the reason I think no one below 18 should be allowed is a simple reason: Maturity. Sure teenagers might be sexually mature and physically mature but they most likely are not socially mature. I myself am sexually mature and not socially mature. I am not sure about physically mature because even though I have been growing at a rate of 1 inch every few years I might get a growth spurt in my 20s. $\endgroup$
    – Caters
    Nov 20, 2016 at 18:11
  • 1
    $\begingroup$ Seems reasonable. $\endgroup$ Nov 20, 2016 at 18:15
  • $\begingroup$ What's the reason to exclude Orphaned? Aliens wants to create drama? And why no pregnant? It's generation ship, pregnancy should be normal. $\endgroup$
    – Mołot
    Nov 21, 2016 at 0:13

3 Answers 3


I would expand the age requirements dramatically. If anything, it may be wise to keep an "old age" requirement based on the rigors of surviving a launch.

This is a generation ship. It's job is to last for generations. In the next 18 years, you are 100% guaranteed to have children on this ship, or you have failed your job. In the next 40 years, you're guaranteed to see elderly people living into their 90s. If you haven't, then you're going to need to make sure there's a system in place to transfer wisdom to replace the wisdom transfer from the elderly to the young.

Generation ships are microcosms, and need to be treated accordingly. If you restrict your crew to 18-50 year olds, you're going to have a really really nasty population inversion issue when those younger people age. You've created an 18 year gap in your workforce. Oops!

  • $\begingroup$ I don't see how that would happen. I mean survival training starts 1 month after the humans board the generation ship and sleep schedule adaptation starts 2 weeks after the humans board the ship. And survival training doesn't take all that long, 1 year on average. After that humans can become pregnant without any consequences. And those of the second generation wouldn't go through survival training until all of them are at least 18. So if anything, it would add more and more young adults and eventually reach equilibrium. $\endgroup$
    – Caters
    Nov 20, 2016 at 21:38
  • 2
    $\begingroup$ @Caters Right, so there will be an 18 year gap in the demographics that will not go away until they convalesce many decades later. When the first children born on the ship hit 18, there will be no adults in the range from 18 to 36, none at all. All having that requirement does is create your generation ship's first major crisis, a 20-30 year region where the demographics are unstable and have trouble supporting the ship. In return you get... well.. nothing really. Just a few years without kids at the start of the journey. $\endgroup$
    – Cort Ammon
    Nov 20, 2016 at 21:40
  • $\begingroup$ So how would adding old people to the requirements change that? The only way I can see of changing that is adding younger people to the requirements and that causes more problems, namely some people being immature and possibly sad to leave their parents. If really young people are allowed on board, some of the women would probably have to induce lactation and then those women who didn't complete survival training but have induced lactation would have to face consequences. $\endgroup$
    – Caters
    Nov 20, 2016 at 21:49
  • $\begingroup$ There's two age issues. The one I was speaking to was the labor gap that appears in the demographics. The other is that our elders carry a great deal of wisdom to be passed on. Chopping that out of our culture would be very rough, because no existing human culture has developed to support the sudden loss of all elders. $\endgroup$
    – Cort Ammon
    Nov 20, 2016 at 21:54
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    $\begingroup$ I think you could get away with a 3 year gap to avoid the lactation issues you mention, but 18 years is a huge imbalance. Its the kind of thing that it'd be nice to ignore, but as the leader of a generation ship, you actually have to consider such realities. $\endgroup$
    – Cort Ammon
    Nov 20, 2016 at 21:55

Your restrictions seem fine and sensible. I would suggest that you might want to add more as currently the alien will still have billions to chose from but restrictiveness is not a problem here. It is nearly impossible to get accurate numbers but I will do my best: There are around 7 billion people in the world however many of these will be too old or young to qualify. Roughly a third of people in the UK are over 50 so if we assume that 2 thirds of the population are out due to age that leaves 4.6 billion people.

In the US over half of the adult population have chronic conditions or are overweight or underweight (http://www.cdc.gov/chronicdisease/overview/). If we take that as a worldwide figure then that eliminates another 2.3 billion leaving 2.3 billion to chose from.

There are around 213 million pregnancies worldwide each year (https://www.guttmacher.org/news-release/2014/new-study-finds-40-pregnancies-worldwide-are-unintended) so we are now down to 2.1 billion.

Roughly 153 million children worldwide are either double or single orphans. If we assume the number is the same in the adult population we are down to 1.9 billion people for the alien to chose from.

All of the above maths is very rough and should not be relied upon but it demonstrates that your alien will have easily enough people to chose from and shows that the restrictions are not too restrictive.


Yes, those requirements are too restrictive.
Or at least, they aren't helpful requirements.

So, Cort Ammon mentioned the generation gap. Your 18-year olds will be 40 before other 18-year olds will be old enough to fill the gap in the workforce, which is tricky but doable. Your fifty year olds will be one hundred before the next generation reaches fifty, which is much less doable. Your young 18-year olds will be at most thirty when the last of the first generation is over fifty (assuming that's a cutoff for a reason), and either there will not be enough to do all the work (because the younger kids are still aging into workforce-age), or you have too many people because there are enough workers but kids (their own replacements) are extra. You need to have replacements ready when someone retires, or dies, or can't do whatever work is needed because of injury or illness - and replacements that take twenty years to arrive are not ideal.

Your first generation must keep working at the same efficiency with no retirement or replacement (while reproducing and raising enough kids to completely replace themselves) for at least twenty years, and even then replacement will be slow, since a generation ship has to control reproduction to not overgrow their resources. Your next generation may be packed into ten years (so as to not have a situation where there aren't enough workers), which can lead to too big a population overlap since they have to reproduce to replace themselves, too. Slow careful reproduction is much more sustainable than this kind of rigid generation gap.

More reasonably, you should plan that kids (and post-retirement elders) will be "extras" - kids not included into the workforce until survival training at eighteen, elders easing off on duties as they can't keep up the level of work. It will let your population adapt to a slow aging in and out of various categories, instead of large overlaps and gaps that will take generations to balance out.

Also, your aliens have some odd prejudices. Restricting overweight and underweight, well, there are some points (especially dealing with medical problems) but it's oddly done - especially without a discussion of what qualifies and how. Varied body types should probably be included in your diversity, especially since the destination environment isn't known. I can understand wanting healthy versions of said body types, just, that is a different requirement than not "underweight" or not "overweight/obese" - especially since mildly overweight is actually a touch healthier than normal weight (because it is aesthetically rather than medically determined), and usual measurements (like the BMI) are notoriously inaccurate.

Also, you mentioned survival training - and the body types that hold on to weight have some actual survival benefit, as a backup energy source and a buffer against lean times or stresses like illness. The tradeoffs of this benefit cause problems in places where overabundance, rather than scarcity, is a problem - but given survival training and an uncertain destination, losing this benefit without discussion may not serve. It may be better to restrict by healthy bodies or active lifestyles (trickier to control for, but better results), rather than policing weight or shape. Restricting by active lifestyle or healthy bodies will also likely leave far fewer people than just restricting to "normal-weight" people, but they will probably be better candidates anyway.

The restriction on orphans is just odd, really - especially since it would necessarily make all your candidates more reluctant to go, knowing they're leaving their families behind! It may disproportionately affect the older demographics on your list, since their parents will be lost to age as well as tragedy. In truth, it might make sense to prefer orphans and/or those without strong familial ties, so that they will have less to regret leaving behind. I could also understand not wanting any children on board to be orphans (in case you relax that restriction), if you're assuming that their parents are on board with them.

As for severe illness and other chronic health conditions, it does make sense to restrict them - especially since it may eliminate some of these conditions in your population. Even restricting those who are actually ill with minor illness might help, since you may prevent those diseases from getting on the ship at all - no guarantees, but it's possible.

And, you should probably consider a few more restrictions - simple things like passing a background check - so not having a criminal record, you probably don't want mass-murderers or serial killers on your ship.

You should prefer people having a good work ethic, and prioritize those having useful knowledge and experience, possibly making an education restriction. You can focus on technical fields (scientists and such), or else craftspeople and artisans, or even survival skills (hey, you're testing for it - might as well screen for it first!)... it depends on the conditions expected on board the ship, what kinds of work will need to be done while the ship is voyaging, and what kinds of conditions are anticipated on the other end of the trip. Knowledge and skills can only be passed down if they made it on the ship to begin with!

You should probably add a requirement for metal health, to go with your requirements for physical health. You should also keep an eye on personality and interpersonal skills, since with such a small population and a long voyage, there will be plenty of room for friction and no where to go to escape people when conflicts happen and drama is everywhere. Much better to minimize that when you can. This may also alleviate some of your concerns about teens not being mature yet - in the beginning, you can select for those that are, at least, better.

You will likely also want to select for those with compatible philosophies, social or political leanings, or whatever equivalents your aliens are going with - so that the population will (hopefully) grow the society they wanted to see in the first place. This would likely be done in the last stage, handpicking from your remaining possibilities, but it probably should be mentioned anyway.


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